In the field of optical communication, a so-called multiplex wavelength transmission is mainly used, in which a plurality of optical signals with different wavelengths are multiplexed and transmitted to one optical fiber, to realize long-distance and mass transmission. Accordingly, it is possible to easily expand transmission capacity on the existing transmission path.
However, it is necessary to make wavelength gaps of optical signals dense, to multiplex a plurality of optical signals within a limited amplification band like a submarine repeater. In this case, interference called coherent crosstalk occurs between the optical signals and the light leaked from the adjacent wavelength signal, and deterioration in signal occurs.
To solve such a problem, a so-called orthogonal polarization multiplex method, in which polarization states of adjacent optical signals are made orthogonal to each other at the time of wavelength multiplexing, is proposed in Patent Citation 1 and the like.
For example, an odd-number channel multiplexer synthesizing odd-number optical signals with different wavelengths to generate multiplex light A that is random polarization light, and an even-number channel multiplexer synthesizing even-number optical signals to generate multiplex light B that is a random polarization wave are provided, the multiplex light A and multiplex light B output from the channel multiplexers are converted into vertical and horizontal linear polarization multiplex light by a multi-channel polarization controller, and the vertical polarization multiplex light and the horizontal polarization multiplex light are synthesized by a polarization synthesizer, thereby generating orthogonal polarization multiplex light.
[Patent Citation 1] JP-A-2004-253931
However, as described in Patent Citation 1, in such an orthogonal polarization multiplex method, optical components are assembled into an apparatus designed to exhibit functions, the positional alignment and adjustment of the optical components are complex and many optical components are coupled using a space optical system, thus there are problems regarding reliability such as temperature stability and long-term stability. In addition, since the apparatus is of a large size, there is a case where the apparatus cannot be housed in a transmission apparatus when the number of channels increases for multiplex wavelengths. Accordingly, the places where the apparatus can be used are limited.
Moreover, it is necessary to use expensive components such as a polarization beam splitter (PBS), an interleaver, array waveguide grating (AWG), a variable optical attenuator (VOA), and thus a cost for producing the apparatus is very high.